This invention relates generally to projection displays including displays using laser and non-laser illumination sources.
Projection displays are subject to so called speckle artifacts under certain circumstances. Speckle is a random spatial interference pattern where the plane wave interferes with itself to modulate its intensity. Speckle may appear like very fine particles sprinkled over a display. The effect is generally static and if the observer and the display are stationary, the effect is visible.
Speckle is largely identified as a problem with coherent light sources, such as lasers. However, incandescent light sources with high gain optics can also produce speckle effects due to optical magnification which produces partial coherence of the light source.
Removing the speckle from a coherent light source is not believed to be possible. Speckle may be avoided in incoherent light sources by avoiding the kind of high gain optics which cause partial coherence.
Speckle effects are dealt with in consumer compact disk (CD) players or CD-ROMs which use solid state laser light sources. Since speckle interferes with reproduction of digital data the current source driving the solid state laser is modulated at very high frequency. This has the effect of spreading out the frequency of light by modulation. While this does not technically eliminate speckle, it does move the speckle around at a high enough frequency to make it invisible to the data detector. This technique does not work with incandescent light sources because one cannot modulate the output light at a sufficiently high frequency.
Projection displays may also suffer from a defect called pixellation. Pixellation is where the "squares" that make up the pixels that form the image become visible. Lines appearing on displays subject to pixellation show jagged effects which appear blocky with additional edges.
Thus, there is a continuing need for improved ways to deal with speckle and pixellation in projection displays.